EPDM vs Felt vs GRP: Choosing a Flat Roof Covering That Actually Works
Extensions, garages and dormer roofs almost always end up flat. Here's how the main covering options compare and where each one makes sense.
Updated 28 July 2026
Why flat roofs need different thinking
A pitched roof sheds water quickly because gravity does most of the work. A flat roof, even one built with a slight fall as it should be, relies far more on the covering itself staying watertight across its whole surface, because water sits and moves slowly rather than running straight off. That difference is why flat roof coverings need to be chosen and installed with more care than people often expect.
Most domestic flat roofs turn up on single-storey extensions, garage roofs, and dormer windows, which is exactly the kind of work we see regularly on the mix of housing across Solihull and Kenilworth, from 1930s semis with later extensions to newer estate builds with flat-roofed garages.
Traditional felt: still around, but limited
Built-up felt roofing, laid in three bonded layers with a torch or hot bitumen, is still fitted on some jobs because it's familiar and cheap in the short term. The problem is that felt has a track record of cracking and splitting as it ages, particularly around joints and upstands, and UV exposure over time makes it brittle faster than most homeowners expect.
Where felt is still the right call is usually on smaller, lower-budget projects like a shed or simple lean-to, where the roof isn't a major feature of the property and a shorter lifespan is an acceptable trade-off for lower cost.
EPDM rubber: the practical choice for most homes
EPDM is a single sheet of synthetic rubber, usually fitted in one piece with minimal joints, which is a real advantage because joints are where flat roofs most often fail. It flexes with temperature changes rather than cracking, which matters in the UK given how much roofs expand and contract between a frosty February morning and a hot July afternoon.
It's also relatively forgiving to install well, which keeps failure rates down compared to felt, and it doesn't need heat or naked flame during fitting, which is a practical safety consideration on a roof close to fences or timber cladding. For most extension and garage roofs we look at across Kenilworth and Solihull, EPDM ends up being the sensible default.
GRP fibreglass: rigid and seamless
GRP is laid as a liquid resin over a fibreglass matting layer and cures into a completely seamless, rigid surface. Because there are no joints at all, it performs well on roofs with a lot of detailing, such as multiple upstands, rooflights or awkward junctions where a sheet material would need extra joints and seams.
It's less forgiving of movement in the roof deck than EPDM, so it suits roofs built on a stable, well-fixed timber or ply deck rather than older structures that flex. Weather during installation matters more too, since it needs dry, reasonably mild conditions to cure properly.
The fall, the deck, and the detailing
Whichever covering you choose, none of it performs well if the roof doesn't have adequate fall to move water toward the outlets or gutter line. A common mistake on older extensions is a roof built dead flat, which almost guarantees ponding and premature failure no matter how good the covering is.
Detailing around the edges matters just as much as the field of the roof. Upstands against walls, roof edge trims, and the junction with guttering all need to be done properly, because these are the points where the vast majority of flat roof leaks actually start.
The short version
- Flat roofs rely entirely on the covering staying watertight, since water doesn't run off quickly.
- Traditional felt is cheaper but more prone to cracking and splitting as it ages.
- EPDM rubber is fitted in one piece with fewer joints, making it a durable choice for most homes.
- GRP fibreglass gives a seamless finish that suits roofs with a lot of upstands or detailing.
- Poor fall and weak edge detailing cause more flat roof leaks than the covering material itself.